Speciation and Formation Constants of Aqueous Lanthanum (III) Chloride Complexes from 5 °C to 80 °C by Raman Spectroscopy

Lanthanum chloride solutions were studied both computationally and by polarized Raman spectroscopy to determine the aqueous species present and their equilibrium constants. Four species were predicted from Gaussian 16 computations using density functional theory: La(H2O)93+, LaCl(H2O)82+, LaCl2(H2O)62+ and LaCl3(H2O)40. Both LaCl(H2O)82+ and LaCl2(H2O)6+ were identified in the reduced isotropic solvent-subtracted Raman spectra from their La—Cl vibrational bands at 234 cm-1 and 221 cm-1, respectively. The spectra did not change significantly with temperature from 5 to 80 °C, and the Raman bands for each species could not be fitted with sufficient accuracy to determine the scattering coefficients directly from the experimental data. Instead, the computed scattering coefficients from Gaussian 16 (B3LYP with the LANL2DZ effective core potential and 6-311+G(d,p) basis set) were to be used to obtain the equilibrium concentrations of LaCl(H2O)82+ and LaCl2(H2O)6+, from which the concentration of La(H2O)93+ was determined. The concentration dependent formation quotients were fitted with the specific ion interaction (SIT) activity coefficient model to determine the stepwise formation constants K0,1 and K1,2, from 5 to 80 °C. Both K0,1 and K1,2 showed minor increases with temperature.